Apparent partial loss age spectra of Neoarchean hornblende (Murmansk Terrane, Kola Peninsula, Russia): the role of biotite inclusions revealed by 40Ar/39Ar laserprobe analysis

Metamorphic hornblende frequently yields spectra with progressively increasing 40Ar/39Ar age steps, often interpreted as caused by partial resetting due to thermally activated radiogenic argon loss by solid-state diffusion. Yet, in many cases rising Ca/K ratio spectra for such samples imply the presence of minor inclusions of K-contaminant minerals. To avoid parts of grains with mineral inclusions or compositional zoning we drilled tiny discs from thin sections under a petrographic microscope. Laser step-heating of drilled biotite-free hornblende discs yielded flat age and ratio spectra. In contrast, furnace step-heated hornblende separates from the same samples produced apparent loss age spectra. Moreover, biotite-free samples yielded flat spectra by laser and furnace dating. Consequently, apparent loss spectra result from degassing of included substantially younger biotite before its hornblende host during laboratory step-heating; c. 2640 Ma hornblende ages constrain the Murmansk Terrane's cooling

Triassic 40Ar/39Ar ages from the Sakaigawa unit, Kii Peninsula, Japan: implications for possible merger of the Central Asian Orogenic Belt with large-scale tectonic systems of the East Asian margin

International audienceThe 218.4 ± 0.4, 228.8 ± 0.9 and 231.9 ± 0.7 Ma 40Ar/39Ar laser probe pseudo-plateau ages (2σ; 49–63% 39Ar-release) of very low-grade meta-pelitic whole-rocks from the Sakaigawa unit date high-P/T metamorphism. We argue that this event occurred in a subduction–accretion complex, not along the East Asian continental margin, but on the Pacific side of the proto-Japan superterrane. Proto-Japan was a Permian magmatic arc, presently dispersed in the Japanese islands, which also contained older subduction–accretion complexes. The arc system was fringing but not yet part of the Eurasian continent. The Middle to Late Triassic high-P/T tectono-metamorphic event was partly coeval with proto-Japan's collision with proto-Eurasia along the southward extension of the Central Asian Orogenic Belt, causing the main metamorphism in the Hida-Oki terrane. It is possible that this system continued via the Cathaysia block (China) to Indochina. The Late Permian to Middle Triassic Indosinian event might stem from docking of Pacific-derived terranes with Southeast Asia's continental margin. The concept of the proto-Japan superterrane implies that the Qinling-Dabie-Sulu suture zone joined the Central Asian Orogenic Belt to the east of the North China craton and did not continue to Japan, as commonly assumed

Apparent partial loss age spectra of Neoarchaean hornblende (Kola Peninsula, Russia): the role of included biotite shown by Ar/Ar laserprobe analysis

Metamorphic hornblende frequently yields spectra with progressively increasing Ar/Ar age steps, often interpreted as caused by partial resetting due to thermally activated radiogenic argon loss by solid-state diffusion. Yet, in many cases rising Ca/K ratio spectra for such samples imply the presence of minor inclusions of K-contaminant minerals. In order to avoid parts of grains with mineral inclusions or compositional zoning we drilled tiny discs from thin sections under a petrographic microscope. Laser step-heating of such micro-sampled biotite-free hornblende discs yielded flat age, Ca/K and Cl/K ratio spectra. In contrast, furnace step-heated hornblende separates from the same samples produced spectra with progressively increasing apparent ages and Ca/K ratios. Biotite-free samples yielded flat age and ratio spectra by both laser and furnace analysis. So, apparent loss spectra result from degassing of included much younger biotite before its hornblende host during laboratory step-heatin

Ordovician 40Ar/39Ar phengite ages from the blueschist-facies Ondor Sum subduction-accretion complex (Inner Mongolia) and implications for the early Paleozoic history of continental blocks in China and adjacent areas

We obtained 453.2 ± 1.8 Ma and 449.4 ± 1.8 Ma (2{sigma}) laser step-heating 40Ar/39Ar plateau ages for phengite from quartzite mylonites from the blueschist-facies Ondor Sum subduction-accretion complex in Inner Mongolia (northern China). These ages are within error of the inverse isochron ages calculated using the plateau steps and the weighted mean ages of total fusion of single grains. The compositional change from glaucophane in the cores to crossite in the rims of blue amphiboles, as revealed by electron microprobe analysis, points to decompression, probably caused by progressive exhumation of the subducted material. The Late Ordovician ages were not affected by excess argon incorporation because in all likelihood the oceanic sediments were wet on arrival at the trench and free of older detrital mica. The ca. 450 Ma ages are, hence, interpreted as the time of crystallization during mylonitization under high fluid activity at fairly low temperatures. This means that accretion of the quartzite mylonite unit occured about 200 Ma before final closure of the Paleo-Asian Ocean, amalgamation of the Siberian, Tarim and North China cratons, and formation of the end-Permian Solonker suture zone. We argue that the Early Paleozoic evolution of the Ondor Sum complex occurred along the northeastern Cimmerian margin of Gondwana, which was composed of micro-continents fringed by subduction-accretion complexes and island arcs. The later evolution took place during the building of the Eurasian continent following middle Devonian and younger rifting along the East Gondwanan margin and northward drift of the detached North China craton. An extensive review shows that this type of two-stage scenario probably also applies to the geodynamic evolution of other micro-continents like, South China, Tarim, a number of Kazakh terranes, Alashan, Qaidam and Kunlun, as well as South Kitakami and correlatives in Japan, and probably Indochina. Like the North China craton, these were bordered by Early Paleozoic subduction-accretion complexes, island arcs or contained calc-alkaline volcanic margins, like for example, the central Tienshan, North Qinling, North Qaidam-Altun, North Qilian and Kunlun belts in China, as well as the Oeyama and Miyamori ophiolites and Matsugadaira-Motai blueschist belt of Japan and the dismembered Sergeevka ophiolite of the southern part of the Russian Far East. This implies that a vast orogenic system, comprising an archipelago of micro-continents, seems to have existed along the Cimmerian margin of East Gondwana in Early Paleozoic time in which the ultrahigh-pressure metamorphism that characterizes the early evolution of many of the Asian micro-continents occurred

L'évolution géodynamique de la chaîne paléozoïque du Tianshan

La chaîne du Tianshan s'étend sur plus de 3000 km en Asie centrale, elle sépare le Tarim au Sud du Juggar et du Kazakhstan au Nord (Fig. 1a). La collision indienne est responsable du haut-relief actuel, mais l'architecture de la chaîne est due à plusieurs événements d'âge Paléozoïque. Classiquement, la chaîne du Tianshan est divisée en Tianshan Nord, Tianshan Central, Tianshan Sud et Bloc de Yili (Fig. 1b). Ce dernier est souvent considéré comme l'extension occidentale du Tianshan Central, mais nos données structurales, géochimiques et paléomagnétiques suggèrent que ces domaines et leurs limites doivent être redéfinis

First age constraints on the timing of metamorphism of the Taean Formation, Anmyeondo: concordant 233 Ma U-Pb titanite and 231-229 Ma 40Ar/39Ar muscovite ages

International audienceIsotopic dating has established that the Middle Paleozoic turbidites of the Taean Formation on Anmyeondo in the West Sea were affected by metamorphism during the Late Triassic. We obtained a 206Pb/238U lower intercept age of 232.5 ± 3.0 Ma (95% confidence, MSWD = 1.2) of metamorphic titanite from a calc-silicate rock by Multi Collector Sensitive High-Resolution Secondary Ion Mass Spectrometry and 40Ar/39Ar laser probe pseudo-plateau ages of 230.7 ± 1.0 Ma and 228.8 ± 1.0 Ma (1σ) for two single grains of metamorphic muscovite. We consider that the metamorphic temperature was below the closure temperatures of titanite and muscovite. Consequently, the concordant U-Pb and 40Ar/39Ar mineral ages are not cooling ages but demonstrate that the metamorphism of the Taean Formation on the island occurred in the earliest Late Triassic (Carnian). The dated muscovite occurs as undeformed grains that cross-cut the main tectono-metamorphic fabric in greenschist facies metapelites, or form undeformed grains in rocks with a well-developed secondary crenulation cleavage. This suggests that the two phases of ductile deformation that affected these Paleozoic sediments occurred earlier. The muscovite age spectra show evidence of an earlier isotopic system of about 237 Ma, which could relate to the observed pre-magmatic folding of the Taean Formation

Tectonic implications of the very fast cooling shown by concordant 230-228 Ma 40Ar/39Ar laser probe hornblende and biotite single grain ages in the Hongseong area

International audienceWe obtained identical 40Ar/39Ar (pseudo) plateau ages of 230.1 ± 1.0 and 229.8 ± 1.0 Ma (1s) on two hornblendes from garnet-bearing corona-textured amphibolites in the Hongseong area. These ages are concordant with the 228.1 ± 1.0 Ma plateau age of biotite in the slightly older amphibolite. The concordant ages of hornblende and biotite, minerals with very different closure temperatures, show that the samples cooled very rapidly, probably in the order of 100-150℃/Ma. The efficiency of cooling is further underlined by the near-coincidence of these 40Ar/39Ar ages with 243-229 Ma (error 2-4%, average: 234.5 Ma) zircon U-Pb ages in the Gyeonggi Massif and the Hongseong belt, reported in the literature. Very fast cooling rates require a fundamental tectonic control. Consequently, we discuss our data in the context of a relatively short-lived, tectonically induced, magmatic and metamorphic pulse that affected the crust in Korea in the Late Triassic. This could have been post-collisional delamination of the lower crust and uppermost mantle, and/or oceanic slab break-off to which the 237-219 Ma mantle-sourced potassic Mg-rich magmatic rocks that are widespread in Korea, also points